1. Field of the Invention
The present invention relates to a control device arranged to control an oxygen sensor for automotive vehicles having a heater. The present invention also relates to an air-fuel ratio controller and an automotive vehicle incorporating such a control device.
2. Description of the Related Art
From the standpoint of environmental issues and energy issues, it has been desired to improve the fuel consumption of internal combustion engines and reduce the emission amount of regulated substances (e.g., NOx) that are contained within exhaust gas from internal combustion engines. In order to meet these needs, it is necessary to appropriately control the ratio between fuel and air during combustion so that fuel combustion will occur always under optimum conditions. The ratio of air to fuel is called the “air-fuel ratio” (A/F). In the case where a ternary catalyst is employed, the optimum air-fuel ratio would be the stoichiometric air-fuel ratio. The “stoichiometric air-fuel ratio” is the air-fuel ratio at which air and fuel will combust just sufficiently.
When fuel is combusting at the stoichiometric air-fuel ratio, a certain amount of oxygen is contained within the exhaust gas. When the air-fuel ratio is smaller than the stoichiometric air-fuel ratio (i.e., the fuel concentration is relatively high), the oxygen concentration in the exhaust gas decreases relative to that under the stoichiometric air-fuel ratio. On the other hand, when the air-fuel ratio is greater than the stoichiometric air-fuel ratio (i.e., the fuel concentration is relatively low), the oxygen concentration in the exhaust gas increases. Therefore, by measuring the oxygen concentration in the exhaust gas, it is possible to estimate the deviation of the air-fuel ratio relative to the stoichiometric air-fuel ratio. This makes it possible to adjust the air-fuel ratio and control the fuel combustion so as to occur under the optimum conditions.
As oxygen sensors for measuring the oxygen concentration within exhaust gas, electromotive force-type oxygen sensors as disclosed in Japanese Laid-Open Patent Publication No. 8-114571 and resistance-type oxygen sensors as disclosed in Japanese Laid-Open Patent Publication No. 5-18921 are known. In order to obtain from an oxygen sensor an accurate sensor output which is in accordance with the oxygen concentration, it is necessary to raise the sensor element to a high temperature (e.g., 300° C. or above). Therefore, commonly-used oxygen sensors include a heater for elevating the temperature of the sensor element. By appropriately controlling the power supplied to the heater with an electronic control unit (ECU) or the like, temperature management for the sensor element is achieved.
An oxygen sensor for measuring the oxygen concentration within the exhaust gas is provided inside an exhaust pipe of an internal combustion engine, and water may accumulate in the exhaust pipe. The reason why water accumulates is that moisture contained in the gas within the exhaust pipe may become condensed after the internal combustion engine is stopped, or that the moisture in the exhaust gas that has passed into the exhaust pipe whose temperature is still low immediately after starting the internal combustion engine may become condensed. If the residual water splatters due to vibration of the internal combustion engine or flow of the exhaust gas, and water droplets adhere to the oxygen sensor whose temperature has been elevated, the oxygen sensor may be destroyed due to thermal shock (called “wet cracking”).
Japanese Laid-Open Patent Publication No. 2004-225617 discloses a technique for preventing destruction of an oxygen sensor due to such wetting. According to this technique, the amount of water accumulating inside the exhaust pipe (residual amount) is inferred, and a treatment for protecting the oxygen sensor is performed in accordance with the inferred residual amount.
Inference of the residual amount is performed from time to time based on the amount of air and the amount fuel supplied to the combustion chamber of the internal combustion engine, external temperature, and the like. Then, if the residual amount is equal to or greater than a reference value, a treatment for reducing the residual amount of water is performed while stopping the power supplied to the heater, and power to the heater is started after the residual amount becomes less than the reference value.
In addition to the aforementioned causes, another cause for the accumulation of water in the exhaust pipe is toppling of a motorcycle. The technique disclosed in Japanese Laid-Open Patent Publication No. 2004-225617 is confined to inferring the amount of water accumulating within the exhaust pipe when the internal combustion engine is normally operated (i.e., the amount of water accumulating due to condensation of moisture within the gas), and does not contemplate water that enters into the exhaust pipe from the outside when a motorcycle topples. An exhaust pipe of a motorcycle is shorter than an exhaust pipe of a four-wheeled automobile, and therefore water having entered from the outside may easily reach the portion where the oxygen sensor is provided.
Many commercially-available motorcycles are equipped with a mechanism for stopping the power of the internal combustion engine and the like (including power to the heater of the oxygen sensor) in case of toppling. However, when power to the heater is resumed upon a restart, wet cracking may occur. Thus, even by using the technique of Japanese Laid-Open Patent Publication No. 2004-225617, wet cracking of an oxygen sensor cannot be sufficiently prevented.
Now, with reference to FIG. 12 to FIG. 17, the reason why the oxygen sensor of a motorcycle is likely to become wet will be described more specifically. As shown in FIG. 12, a motorcycle 500 includes an internal combustion engine 514 and an exhaust pipe 523 which is connected to the internal combustion engine 514. The motorcycle 500 further includes a muffler 524 for reducing exhaust noise.
FIG. 13 is a diagram schematically showing the internal combustion engine 514, the exhaust pipe 523, and the muffler 524 of the motorcycle 500, in particular. FIG. 14 is a cross-sectional view taken along line 14A-14A′ in FIG. 13. As shown in FIG. 13, an oxygen sensor 510 for measuring the oxygen concentration in the exhaust gas is attached to the exhaust pipe 523. Although not shown herein, the top end of the oxygen sensor 510 is exposed within the exhaust pipe 523. The rear (downstream side) portion of the exhaust pipe 523 is accommodated in the muffler 524.
The inside of the muffler 524 is divided into a primary expansion chamber 524a and a secondary expansion chamber 524b by a partition 525, respectively, from the front (upstream side) side. The exhaust pipe 523 penetrates the partition 525, such that the downstream side end of the exhaust pipe 523 opens within the secondary expansion chamber 524b. 
The primary expansion chamber 524a and the secondary expansion chamber 524b are in communication with each other via a communication pipe 526 which is provided so as to penetrate the partition 525. A tail pipe 527 is provided in the secondary expansion chamber 524. The tail pipe 527 penetrates the partition 525 such that the upstream side end thereof opens within the primary expansion chamber 524a. The tail pipe 527 also penetrates an outer wall 524w of the muffler 524 such that the downstream side end thereof is open outside the muffler 524. Exhaust gas which is discharged from the exhaust pipe 523 into the secondary expansion chamber 524b is introduced into the primary expansion chamber 524a through the communication pipe 526, and further passes through the tail pipe 527 so as to be externally discharged. In the primary expansion chamber 524a of the muffler 524, a drain hole 524h (which is shown in FIG. 13 larger than actual size for ease of understanding) is provided for externally discharging the water which has become condensed within the muffler 524.
FIG. 15 is a diagram showing the motorcycle 500 having toppled on the ground which is covered with water. FIG. 16 is a diagram showing a cross-section of the muffler 524 (corresponding to FIG. 14) at this time. As shown in FIG. 15, when the motorcycle 500 is toppled, water comes into the muffler 524 through the tail pipe 527 as shown in FIG. 16, so that the water accumulates inside the muffler 524. Although the muffler 524 has the drain hole 524h as shown in FIG. 13, the drain hole 524h only has a small radius (with a diameter of about 1 to 2 mm) in order to prevent a large amount of exhaust gas from leaking therethrough, and therefore does not have the ability to sufficiently discharge the water which has flowed into the muffler in the case of toppling.
When the body of the motorcycle 500 is set upright with the water accumulated within the muffler 524, as shown in FIG. 17, the water inside the muffler 524 flows toward the upstream side of the exhaust pipe 523. If a large amount of water flows into the exhaust pipe 523, the water may reach the portion of the oxygen sensor 510 protruding into the exhaust pipe 523. Therefore, when power to the heater is resumed at the time of a restart, wet cracking may occur.